In modern process and automation technology, it is known, for example, to measure fill level via the capacitive measuring principle. In such case, a probe unit and the wall of the container, or a second probe unit, form, in combination with the medium as dielectric, a capacitor. The capacitance of this capacitor depends on the fill level of the medium. Thus, it is possible, based on the measured capacitance of the capacitor, to ascertain, or monitor, as the case may be, fill level. A problem, in this connection, is presented by the fact that the medium comes in contact with the probe unit, and, thus, can adhere to the probe unit. In the presence of such accretion, measuring, or monitoring, of fill level is generally no longer possible. In view of this, for example, probe units are used, which include a probe electrode and at least one guard electrode. The guard electrodes are, in such case, most often, supplied with the same signal as the probe electrode, so that the two electrodes lie at the same electrical potential. Such guard electrodes are, above all, advantageous, when accretion of medium leads to a connection between the probe and the container wall. The electric field occurring as a result of the guard electrode effectively prevents electrical current flow from the probe electrode through the accretion to the container wall. Especially in the case of conductive, adhering media, it is necessary to work with such a guard electrode, in order to have a safer, measured value. The better the electrical coupling of the guard electrode to the adhering medium, the better is the guard effect. From this, it follows that a bare guard electrode would best fulfill the function. In the case of applications, where the medium is food, it is, however, for hygienic reasons, required, that a “gap-free construction” be assured. In such case, a probe unit with bare guard electrode and insulated probe electrode (this prevents direct electrical contact between probe and medium) is unavoidably associated with a gap, since, here, the material pairing of metal and plastic excludes the possibility of materially bonded connection. A completely insulated arrangement, in the case of which the insulation surrounds both the probe electrode and the guard electrode, does not have this disadvantage of the missing gap-free connection.
For a high accretion insensitivity with a gap-free, fully insulated probe unit, thus, there are the following requirements for the application of the guard electrode: The thickness of the insulation in the region of the guard electrode should be as small as possible; furthermore, the DK value—thus, the value of the dielectric constant of the insulation in the same region—should be as high as possible, because the value of the impedance between the guard electrode and the surroundings should be as small as possible.